“Imperial” grade natural jadeite gems are extremely rare and highly valued with prices exceeding those of equivalently sized gem diamonds. The term “jade” as used in the gem trade refers to two minerals: jadeite (NaAlSi2O6) and nephrite (Ca2(Mg,Fe)5Si8O22(OH)2). Before the 18th century, only nephrite had been found, mined, and carved into art objects or jewelry. During the 1700's, a new mineral, jadeite, was found in Burma. Jadeite is different in chemical composition from nephrite and can exhibit highly saturated colors and a higher degree of translucency, a higher hardness, and a glassy appearance. Because of its superior properties, jadeite quickly replaced nephrite as the “jade” of choice among art and gem collectors. Contemporary descriptions of “Imperial” jadeite cite the following qualitative attributes: a very intense and uniform green color ranging from “apple green” to “spinach green”; a high degree of uniform translucency; and an extremely smooth finish with a greasy feel and luster. The surfaces of natural gems often are treated with green waxes to improve color and translucency by filling surface defects.
Jadeite is a high-pressure polycrystalline mineral with nominal chemical composition of NaAlSi2O6 and a density of 3.3 g/cc. It occurs naturally in a variety of colors ranging from colorless or white to yellowish white, green, lavender, red, and black. A specific green hue is associated with “Imperial” jade and has been highly prized since its discovery. The color of jadeite is due to the presence of impurity elements. For example, the presence of iron and/or chromium imparts a green color to the mineral. Manganese provides a lavender color.
Since the 1950's, jadeite has been synthesized in laboratory quantities in high-pressure (HP) phase equilibrium experiments to elucidate planetary evolution. Temperatures over 600° C. (for hydrothermal synthesis) and pressures over 20 Kbar were required. These numerous works produced very small laboratory samples, typically less than 2 mm in any dimension and which were of no gemological value. A much smaller number of reports describe attempts to produce jadeite in gemologically valuable size and quality. These reports reveal that larger jadeite samples can be produced with nominal mineralogical attributes of density, refractive index, hardness, and crystallographic structure. None of these reports quantitatively describes the product or claims to synthesize “Imperial” grade jadeite. In fact, the most detailed evaluations of synthetic jadeite categorically state that “Imperial” quality was not achieved.
Shigley and Nassau, (Kurt Nassau and James E. Shigley, “A Study of the General Electric Synthetic Jadeite”, Gems & Gemology, Spring 1987, pp. 27-35) provided the most complete gemological evaluation of synthetic jadeite. They report on several samples produced by high-pressure processes. The gemologists found that the synthetic materials were predominately jadeite and reproduced the chemical composition, refractive index, density, infrared spectra, x-ray patterns, fluorescence, and hardness values expected of natural jadeite. Samples up to 2.6 carats in weight were described. The authors also indicated that while this jadeite, “can be considered gem material, it does not match the highly translucent, almost transparent, quality of what is known in the trade as ‘Imperial’ jadeite',” and described the material as, “semi-translucent to opaque.” The authors attributed the opacity to the presence of a minor concentration of glassy phase. They further noted that the color was concentrated in distinct areas providing a “mottled” and “granular” appearance. Lamellar cracks, unexpected trace contaminants, non-uniform polishing, and an unusual reflective property called “adventuresence” further distinguished the synthetic product from “Imperial” gem quality jadeite. FIG. 1, shows the lack of uniformity, cracking, and opacity of the materials evaluated by Shigley.
In two publications DeVries and Fleischer (DeVries and Fleischer, “Synthesis of Jadeite for Jewelry”, GE Technical Information Series, 84 CRD 282, 1984; and DeVries and Fleischer, Material Research Society Symp. Proc. Vol 22, pp. 203-207, 1984, Elsevier Science Pub. Inc.), who successfully synthesized jadeite minerals more than 17 years ago, noted that, while of gemologically useful size, the “quality of Imperial jade is not achieved.” The transparency of their products was limited by the presence of a residual eutectic material. Microgaphs in their reports show this second phase to be present between 5 and 10 volume percent and up to 30 microns in size. These authors further describe non-uniform coloration as “mottled” and “mixed.” Radial inhomogeneity and lateral cracking limited the size of gems that could be fabricated. While not cited by these authors as limiting transparency, the synthetic jadeite microstructures shown exhibited other defects: intergranular cracking of up to 30% of the grain boundaries; polishing pullout; and large, distinct grains averaging 30 microns in diameter and as large as 150 microns.
More recently, Zhao, et al. (Tinghe Zhao, et al., “The Physical and Chemical Properties of Synthetic and Natural Jade for Jewellery”, J. Material Science, vol 29, 1996 pp. 1514-1520; Zhao, et al., “Synthesis of the clinopyroxenes CaMgSi2O6—NaAlSi2O6 for jewelry”, J. Material Science, 30, 1995, pp. 1117-1123) describe an expanded range of process parameters used to synthesize gem-sized pieces of jadeite. The mineralogical and compositional properties of natural jadeite were achieved on samples twice the size of the DeVries materials. The largest sample prepared was approximately 15 carats in weight. The gemological quality of the samples is not described. These authors characterize the products as, “green,” “emerald green,” and “translucent.” In the second reference, other colors were produced, but not in stoichiometric jadeite. In this second article, the translucency was described as “somewhat improved” but not measured. The jadeite microstructure in both Zhou references contains fibrous, elongated crystals up to 10 by 40 microns with very distinct grain boundaries.
With respect to the processes used by the foregoing authors to make synthetic Jadeite, DeVries and Fleischer used glass powders having the composition, NaAlSi2O6, and containing small amounts (0.5 to 2.0 weight-%) coloring agents, such as Cr2O3 for green and manganese oxides for lavender. The glass compositions were melted and crushed to achieve homogeneity. Crushed glass powders (−60/+100 mesh size) were placed in a high-pressure cell in direct contact with the heating element. The powders were sintered and annealed in the jadeite stable region of the pressure-temperature phase diagram. The mineralogical properties of jadeite were routinely obtained in this work. The samples produced were of poor quality and there was high incidence of cracking and delamination of the samples. This is attributed to the large volume reduction during sintering of glass powder into a dense jadeite sample. The packing density of glass powder is only ˜60% of theoretical density of solid glass. Further, the jadeite glass phase itself has just 74% of the density of the crystalline jadeite phase of corresponding composition. Hence, sintering of jadeite glass powder to a jadeite crystal resulted in very large combined (60%) reduction in part volume during high-pressure processing. This large volume reduction causes distortion of the cell making the process less reliable, causing cracking, and limiting the size of samples that can be made by powder sintering. The authors attributed cracking and radial non-uniformities to the use of indirect heating. The cell used a graphite heater, which was in direct contact with the samples.
Zhao et aL's process also used a similar glass powder approach to make jadeite and jadeite-like clinopyroxenes by sintering at jadeite stable high-pressure and high temperature conditions. These investigators used a direct contact, indirect heated cell almost identical to the DeVries design. The quantitative details on the quality of the samples are not available other than that they were translucent and green in color.
These contemporary descriptions of synthetic jadeite qualitatively describe color, uniformity, translucency, and finish. None claim to have synthesized “Imperial” grade material. The most detailed reports categorically state that the “Imperial” quality level was not achieved.
Thus, despite the reported ability to synthesize jadeite under high-pressure processing conditions, no reports of Imperial jadeite have been published, regardless of size of the resulting jadeite synthesized. Thus, there exists a need in the art to manufacture jadeite of improved quality and size. There also is a need in the art to manufacture Imperial jadeite. The present invention is addressed to these needs.